finidat = '$path_to_init_file' use_init_interp = .true.
Publicly available initialization files from CESM2 are available, check out this page: Restart Files
Initialization files are written out from a previous run CESM1 simulation and specify initial states of chemical fields, instead of spinning up from zero. Another option is to use the default initialization file, perform a one-year spin up and re-initialize with the output from that simulation to minimize the initial condition influence.
Every year from 1975-1993: /home/buchholz/CESM206_CAMchem_longrun_fmerra
We will have a set of publicly available initialization files soon. For now, either access from Cheyenne HPSS or contact someone in the CAM-chem team if you wish to use something other than the default file.